1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor film having a crystal structure and formed on a substrate having an insulating surface, and a method of fabricating a semiconductor device using the semiconductor film as an active layer. Particularly, the present invention relates to a method of fabricating a thin film transistor in which an active layer is formed of a crystalline semiconductor layer. Incidentally, in the present specification, the term xe2x80x9csemiconductor devicexe2x80x9d indicates all devices capable of functioning by using semiconductor characteristics, and includes, in its category, an electro-optical device typified by an active matrix type liquid crystal display device formed by using thin film transistors, and an electronic equipment incorporating that kind of electro-optical device as a part.
2. Description of the Related Art
There has been developed a thin film transistor (hereinafter referred to as a TFT) in which an amorphous semiconductor layer is formed on a translucent substrate having an insulating surface and a crystalline semiconductor layer crystallized by a laser annealing method, heat annealing method or the like is made an active layer. As the insulating substrate, a glass substrate of barium borosilicate glass or alumino borosilicate glass is often used. Although such a glass substrate is inferior to a quartz substrate in heat resistance, it has merits that its market price is inexpensive and a large area substrate can be easily manufactured.
The laser annealing method is known as a crystallizing technique in which it is possible to crystallize an amorphous semiconductor layer by giving high energy to only the amorphous semiconductor layer without raising the temperature of a glass substrate very much. Particularly an excimer laser capable of obtaining short wavelength light having a wavelength of 400 nm or less and large output is regarded as most suitable in this usage. The laser annealing method using the excimer laser is carried out in such a manner that a laser beam is processed by an optical system into a spot shape or linear shape on a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the processed laser beam (irradiation position of the laser beam is moved relatively to the surface to be irradiated). For example, in an excimer laser annealing method using a linear laser beam, it is also possible to make laser annealing of all the surfaces to be irradiated by scanning only in the direction normal to its longitudinal direction, and is superior in productivity, so that it has become the mainstream of a manufacturing technique of a liquid crystal display device using TFTs. The technique enables a monolithic type liquid crystal display device in which TFTs (pixel TFTs) for forming a pixel portion and TFTs of a driving circuit provided at the periphery of the pixel portion are formed on one glass substrate.
However, a crystalline semiconductor layer fabricated by the laser annealing method is formed of an aggregation of plural crystal grains, and the positions and sizes of the crystal grains are random. TFTs fabricated on the glass substrate are formed such that the crystalline semiconductor layer is separated into an island-like pattern for the purpose of element separation. In that case, it was impossible to specify the positions and sizes of the crystal grains and form them. In the interface (crystal grain boundary) of the crystal grain, there is a cause to lower current transport characteristics of carriers because of a recombination center or trapping center due to an amorphous structure, crystal defect or the like, or the influence of a potential level at the crystal grain boundary. However, it has been hardly possible to form a channel formation region, in which the property of a crystal greatly influences the characteristics of a TFT, by a single crystal grain so as to exclude the influence of the crystal grain boundary. Thus, a TFT including an active layer of a crystalline silicon film and having characteristics comparable to those of a MOS transistor has not been obtained till today.
In order to solve such problems, an attempt to grow a large crystal grain has been made. For example, in ┌xe2x80x9cHigh-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Methodxe2x80x9d, K. Shimizu, O. Sugiura, and M. Matsumura, IEEE Transactions on Electron Devices vol. 40. No. 1, pp 112-117, 1993┘, there is a report on a laser annealing method in which a film of three-layer structure of Si/SiO2/Si is formed on a substrate, and an excimer laser beam is irradiated from both sides of a film side and a substrate side. This report discloses that according to this method, the size of a crystal grain can be enlarged by irradiation of a laser beam at predetermined energy intensity.
The above-mentioned method of Ishihara et al. is characterized in that heat characteristics of an under material of an amorphous silicon film are locally changed and the flow of heat to the substrate is controlled, so that a temperature gradient is caused. However, for that purpose, the three-layer structure of high melting point metal layer/silicon oxide layer/semiconductor film is formed on the glass substrate. Although it is possible to form a top gate type TFT by using the semiconductor film as an active layer in view of structure, since a parasitic capacitance is generated by the silicon oxide film provided between the semiconductor film and the high melting point metal layer, power consumption is increased and it becomes difficult to realize high speed operation of the TFT.
On the other hand, when the high melting point metal layer is made a gate electrode, it is conceivable that the method can be effectively applied to a bottom gate type or reverse stagger type TFT. However, in the foregoing three-layer structure, even if the thickness of the semiconductor film is omitted, with respect to the thickness of the high melting point metal layer and the silicon oxide layer, since the thickness suitable for a crystallizing step is not necessarily coincident with the thickness suitable for the characteristics as a TFT element, it is impossible to simultaneously satisfy both the optimum design in the crystallizing step and the optimum design in the element structure.
Besides, when the opaque high melting point metal layer is formed on the entire surface of the glass substrate, it is impossible to fabricate a transmission type liquid crystal display device. Although the high melting point metal layer is useful in that its thermal conductivity is high, since a chromium (Cr) film or titanium (Ti) film used as the high melting point metal material layer has high internal stress, there is a high possibility that a problem as to adhesiveness to the glass substrate occurs. Further, the influence of the internal stress is also exerted on the semiconductor film formed as the upper layer, and there is a high possibility that the stress functions as force to impart distortion to the formed crystalline semiconductor film.
On the other hand, in order to control a threshold voltage (hereinafter referred to as Vth) as an important characteristic parameter in a TFT within a predetermined range, in addition to valence electron control of the channel formation region, it is necessary to reduce the charged defect density of a base film and a gate insulating film formed of an insulating film to be in close contact with the active layer, or to consider the balance of the internal stress. To such requests, a material containing silicon as its constituent element, such as a silicon oxide film or a silicon nitride oxide film, has been suitable. Thus, there is a fear that the balance is lost by providing the high melting point metal layer to cause the temperature gradient.
The present invention has been made to solve such problems, and an object of the invention is to realize a TFT capable of operating at high speed by fabricating a crystalline semiconductor film in which the positions and sizes of crystal grains is controlled and further by using the crystalline semiconductor film for a channel formation region of the TFT. Further, another object of the invention is to provide a technique enabling such a TFT to be applied to various semiconductor devices such as a transmission type liquid crystal display device or a display device using organic electroluminecence material.
A laser annealing method is used as a method of forming a crystalline semiconductor layer from an amorphous semiconductor layer formed on a substrate of glass or the like. In the laser annealing method of this invention, a pulse oscillation or continuous-wave excimer laser or argon laser is used as a light source, and a laser beam formed into a linear shape by an optical system is irradiated to an island-like semiconductor layer from both sides of a front side of a substrate where the island-like semiconductor layer is formed (in this specification, the front side is defined as a surface where an island-like semiconductor layer is formed) and a reverse side (in this specification, it is defined as a surface opposite to the surface where the island-like semiconductor layer is formed).
FIG. 2A is a view showing a structure of a laser annealing apparatus of the present invention. The laser annealing apparatus includes a laser oscillator 1201, an optical system 1100, and a stage 1202 for fixing a substrate. The stage 1202 is provided with a heater 1203 and a heater controller 1204, and can heat the substrate up to 100 to 450xc2x0 C. A reflecting plate 1205 is provided on the stage 1202, and a substrate 1206 is set thereon. In the structure of the laser annealing apparatus of FIG. 2A, a method of holding the substrate 1206 will be described with reference to FIG. 2B. The substrate 1206 held at the stage 1202 is set in a reaction chamber 1213, and is irradiated with a laser beam. The inside of the reaction chamber can be made a low pressure state or inert gas atmosphere by a not-shown exhaust system or gas system, and a semiconductor film can be heated up to 100 to 450xc2x0 C. without pollution. The stage 1202 can be moved along a guide rail 1216 in the reaction chamber, and the entire surface of the substrate can be irradiated with the linear laser beam. The laser beam is incident from a not-shown quartz window provided above the substrate 1206. Besides, in FIG. 2B, a transfer chamber 1210, an intermediate chamber 1211, and a load/unload chamber 1212 are connected to the reaction chamber 1213, and they are separated by partition valves 1217 and 1218. A cassette 1214 capable of holding a plurality of substrates is set in the load/unload chamber 1212, and the substrate is conveyed by a conveying robot 1215 provided in the transfer chamber 1210. A substrate 1206xe2x80x2 indicates a substrate under conveyance. By adopting such structure, it is possible to continuously carry out laser annealing under the low pressure or in the inert gas atmosphere.
FIGS. 3A and 3B are views for explaining the structure of the optical system 1100 of the laser annealing apparatus shown in FIG. 2A. An excimer laser, argon laser or the like is used as a laser oscillator 1101. FIG. 3A is a view of the optical system 1100 seen from the side, and a laser beam emitted from the laser oscillator 1101 is divided in the vertical direction by a cylindrical lens array 1102. After the divided laser beam is once condensed by a cylindrical lens 1104, it broadens, is reflected by a mirror 1107, and then, is made a linear laser beam on an irradiation surface 1109 by a cylindrical lens 1108. By this, the energy distribution of the linear laser beam in a width direction can be uniformed. FIG. 3B is a view of the optical system 1100 seen from above, and the laser beam emitted from the laser oscillator 1101 is divided in the horizontal direction by the cylindrical lens array 1102. Thereafter, the laser beams are synthesized into one beam on the irradiation surface 1109 by the cylindrical lens 1105. By this, the energy distribution of the linear laser beam in the longitudinal direction can be uniformed.
FIG. 1 is a view for explaining the concept of a laser annealing method of the present invention, An insulating film 1002 is formed on a substrate 1001 of glass or the like, and an island-like semiconductor layer 1003 is formed thereon. A silicon oxide film, a silicon nitride film, a silicon nitride oxide film, an insulating film containing aluminum as its main ingredient, or the like is applied to the insulating film 1002, and a single film among these or a suitable combination of these is used. By the optical system explained in FIGS. 3A and 3B, the laser beam having passed through the cylindrical lens 1005 with the function equivalent to the cylindrical lens 1108 is irradiated as the linear laser beam to the island-like semiconductor layer 1003. The island-like semiconductor layer 1003 receives a first laser beam component 1006 which passes through the cylindrical lens 1005 and is directly irradiated to the island-like semiconductor layer 1003 and a second laser beam component 1007 which passes through the insulating film 1002 and the substrate 1001, is reflected by a reflecting plate 1004, again passes through the substrate 1001 and the insulating film 1002, and is irradiated to the island-like semiconductor layer 1003. In any case, since the laser beam having passed through the cylindrical lens 1005 has an incident angle of 45 to 90xc2x0 with respect to the surface of the substrate in the condensing process, the laser beam reflected by the reflecting plate 1004 is also reflected toward the inside of the island-like semiconductor layer 1003. In the reflecting plate 1004, a reflecting surface is formed of aluminum (Al), titanium (Ti), titanium nitride (TiN), chromium (Cr), tungsten (W), tungsten nitride (WN), or the like. Like this, by suitably selecting the material forming the reflecting surface, the reflectivity can be changed in the range of 20 to 90%, and the intensity of the laser beam incident from the reverse side of the substrate 1001 can be changed. If the reflection surface is made a mirror surface, positive reflectivity of about 90% can be obtained within the wavelength range of 240 to 320 nm. Besides, if the material is made aluminum and minute uneven shapes of several hundred nm are formed on the surface, diffusion reflectivity (integral reflectivityxe2x80x94positive reflectivity) of 50 to 70% is obtained.
In this way, the laser beam is irradiated from the front surface and the reverse surface of the substrate 1001, and the island-like semiconductor layer formed on this substrate 1001 is laser annealed from both surfaces. In the laser annealing method, by optimizing the condition of an irradiated laser beam, a semiconductor film is instantaneously heated and melted, and the generation density of crystal nuclei and crystal growth from the crystal nuclei is controlled. Since the oscillation pulse width of an excimer laser is several nano seconds to several tens nano seconds, for example, 30 nano seconds, if irradiation is made under a pulse oscillation frequency of 30 Hz, the semiconductor layer of the region which is irradiated with the laser beam is instantaneously heated by the pulse laser beam, and is cooled for a time far longer than the heating time.
If the laser beam is irradiated to the island-like semiconductor layer formed on the substrate from only one surface, only one side is heated, so that a cycle of heating melting and cooling solidification becomes steep. Thus, even if the generation density of crystal nuclei can be controlled, satisfactory crystal growth can not be expected. However, if the laser beam is irradiated from both surfaces of the semiconductor layer, the cycle of heating melting and cooling solidification becomes gentle, and a time allowed for crystal growth in the process of cooling solidification becomes relatively long, so that satisfactory crystal growth can be obtained.
In the transient phenomenon, an attempt is made such that the island-like semiconductor layer is made to have a temperature distribution, a region where temperature change is gentle is formed, and a nucleus generation speed and nucleus generation density are controlled, so that the size of the crystal grain is enlarged. Specifically, as shown in FIG. 1, in the region where the island-like semiconductor layer 1003 overlaps with the base film 1002, a thick portion is formed in the base film 1002. At this portion, since its volume is increased and heat capacity is increased, the cycle of temperature change of the island-like semiconductor layer by the irradiation of the pulse laser beam becomes gentle (as compared with the other thin portion). In the present invention, like this, the laser beam is irradiated from the front surface side and the reverse surface side of the substrate to directly heat the semiconductor layer, and at the same time, heat conduction control from the semiconductor layer to the substrate side and heat conduction (due to a temperature gradient) of the semiconductor layer in the horizontal direction to the substrate are used, so that enlargement of the size of the crystal grain is realized.
In addition, with respect to the method of irradiating the laser beam from the front surface side and the reverse surface side of the substrate on which the island-like semiconductor layer is formed, a structure shown in FIG. 4 may be used. A light beam emitted from a laser oscillator 401 such as an excimer laser is divided by a cylindrical lens array 402 (or 403). After this divided laser beam is once condensed by a cylindrical lens 404 (or 405), it broadens and is reflected by a mirror 408. A beam splitter 406 is put on the midway of this optical path to divide the optical path in two. One laser beam is reflected by mirrors 407 and 413, is made a linear laser beam by a cylindrical lens 414, and is irradiated to the front side of a substrate 418. This laser beam is made a first laser beam. A base film 419 and an island-like semiconductor layer 420 are formed on the front side of the substrate 418. The other laser beam is reflected by mirrors 408, 409 and 411, is made a linear laser beam by a cylindrical lens 412, and is irradiated to the reverse side of the substrate 418. This laser beam is made a second laser beam. In the midway of this optical path, an attenuator is provided to adjust the intensity of the laser beam. In this structure, even when the laser beam is irradiated from the front side and the reverse side of the substrate, the size of the crystal grain of the semiconductor layer can be enlarged similarly to the foregoing.
In this invention, such a laser annealing method is called a dual beam laser annealing method, and this method is used to enlarge the size of a crystal grain of an island-like semiconductor layer. Such an island-like semiconductor layer is used for an active layer of a TFT, and further, a semiconductor device including a TFT having a structure in accordance with the function of each circuit is fabricated, so that the performance of the semiconductor device is improved.
The structure of the present invention using such a dual beam laser annealing method is characterized in that a base film having a region of a first thickness and a region of a second thickness smaller than the first thickness are formed on one surface of a translucent substrate, the region of the first thickness has an area smaller than the region of the second thickness, and an island-like semiconductor layer having a crystal structure on the base film is formed over the region of the first thickness and the region of the second thickness.
Another structure of the invention is characterized in that a heat conduction layer formed like an island is provided on one surface of a translucent substrate, a base film on the translucent substrate is formed to cover the heat conduction layer, and at least a part of an island-like semiconductor layer having a crystal structure on the base film is formed on the heat conduction layer.
Besides, another structure of the present invention is characterized by including a step of forming a base film of a first thickness on one surface of a translucent substrate, a step of forming a region of the first thickness and a region of a second thickness smaller than the first thickness by etching a part of the base film, a step of forming an island-like semiconductor layer on the base film and over the region of the first thickness and the region of the second thickness, and a step of crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from one surface side and the other surface side of the translucent substrate.
Besides, another structure of the present invention is characterized by including a step of forming an island-like heat conduction layer on one surface of a translucent substrate, a step of forming a base film of a first thickness on the translucent substrate to cover the island-like heat conduction layer, a step of forming an island-like semiconductor layer which is formed on the base film, which has an area larger than the island-like heat conduction layer, and at least a part of which overlaps with the island-like heat conduction layer, and a step of crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from one surface side of the translucent substrate and the other surface side.
Besides, another structure of the present invention is characterized by including a step of forming a base film of a first thickness on one surface of a translucent substrate, a step of forming a region of a first thickness and a region of a second thickness smaller than the first thickness by etching a part of the base film, a step of forming an island-like semiconductor layer on the base film and over the region of the first thickness and the region of the second thickness, and a step of crystallizing the island-like semiconductor layer by irradiating the laser beam from one surface side of the translucent substrate and by causing a reflecting plate provided at the other surface side of the translucent substrate to reflect a laser beam, which was incident on a peripheral region of the island-like semiconductor layer and passed through the translucent substrate, so that the laser beam is irradiated from the other surface side of the translucent substrate.
Besides, another structure of the present invention is characterized by including a step of forming an island-like heat conduction layer on one surface of a translucent substrate, a step of forming a base film of a first thickness on the translucent substrate to cover the island-like heat conduction layer, forming an island-like semiconductor layer which is formed on the base film, which has an area larger than the island-like heat conduction layer, and at least a part of which overlaps with the island-like heat conduction layer, and a step of crystallizing the island-like semiconductor layer by irradiating a laser beam from one surface side of the translucent substrate and by causing a reflecting plate provided at the other surface side of the translucent substrate to reflect a laser beam, which was incident on a peripheral region of the island-like semiconductor layer and passed through the translucent substrate, so that the laser beam is irradiated from the other surface side of the translucent substrate.